Disc drive for an optical scanning device

ABSTRACT

The invention relates to an optical disc drive that comprises rotating means, defining a rotating axis for an optical disc, and an optical scanning device, for scanning said optical disc with a light beam. These optical scanning means themselves comprise a first light source, for producing said first light beam, focusing means, provided between the first light source and a focusing point on an information layer on the disc, an optical detector, for receiving a first backward beam reflected from said information layer, and a second light source, for producing a second light beam also transmitted to said focusing means and for measuring tilt from the position, on said optical detector, of a second spot corresponding to a second backward beam obtained after reflection of the second light beam on the information layer. According to the invention, a diffractive structure, provided with diffracting elements for substantially refocusing the returning second beam onto the detector, is provided between said focusing point and said optical detector. This diffractive structure, consisting of a series of ring-shaped prisms, is attached either to one surface of a servo-lens positioned just before said optical detector, or to one surface of an objective lens used as focusing means, or to a separate plate. The diffractive structure may also be approximated by a step-wise structure.

FIELD OF THE INVENTION

The present invention generally relates to optical scanning devices forstoring information onto a disc-shaped storage medium or readinginformation from such disc-shaped storage medium, where the disc isrotated and a write/read head is moved radially with respect to therotating disc. More particularly, the invention relates to an opticaldisc drive comprising rotating means, defining a rotating axis for anoptical disc, and optical scanning means, for scanning said optical discwith a light beam.

The present invention is applicable in the case of optical ormagneto-optical disc systems. Hereinafter, the wording “optical discsystem” will be used, but it is to be understood that this wording isintended to also cover magneto-optical disc systems.

BACKGROUND OF THE INVENTION

It is commonly known that an optical storage disc comprises at least onetrack of storage space where information (such as images, data, music, .. . ) may be stored. Optical storage discs may be of a read-only type,where information is recorded during manufacture and can only be read bya user, or of a writable type, where information may be stored by auser. For writing information in the storage space of the opticalstorage disc or for reading information from the disc, an optical discdrive comprises, on the one hand, rotating means for receiving androtating the optical disc and, on the other hand, optical means forgenerating an optical beam, typically a laser beam, and scanning thestorage track with said laser beam. The rotating means are for example amotor, which drives a hub engaging a central portion of the opticaldisc. For optically scanning the rotating disc, the optical disc drivecomprises for instance at least a light beam generator device (typicallya laser diode), an objective lens, for focusing the light beam in afocal spot on the information layer of the disc, and an opticaldetector, for receiving the reflected light reflected from the disc andgenerating an electrical detector output signal. Since the technology ofoptical discs in general, the way in which information can be stored inan optical disc and the way in which optical data can be read from anoptical disc are commonly known, it is not necessary here to describethis technology in more detail.

In an ideal case, the information layer of the disc is assumed to beperpendicular to the optical axis of the apparatus, so that thereturning beam, i.e. the light reflected from the disc, is co-axial andcounter propagates to the incoming light. In the case where the disc hasa warped surface, the propagation axis of the reflected light is nolonger co-axial to the axis of the incoming light, and a tilt effect,consisting in a deviation of the returning beam from the co-axialpropagation of the ideal case, is observed. Such a disc-tilt is known todegrade the performance of optical disc-drives and has to be detectedand compensated. Nevertheless, since the incoming light is focused to atiny spot on the information layer of the disc, this deviation fromco-axial propagation is very small and a tilt detector based on thisprinciple of detection of said deviation is very difficult to implement.

It has then been proposed to amplify the deviation of the returningbeam, for instance by adding aberrations to the incoming beam andtherefore causing the spot size on the information layer to increase.For instance, when the disc is accessed with infra-red light(wavelength=about 790 nm), the larger amount of spherical aberration(said aberration being due to the fact that, in a radiation beam, therays in the central part of the beam and the rays of the periphery ofthe beam have different focal points when they are focused on the disc)causes the spot size on the information layer of the disc to increasecompared to the situation where the disc is accessed with red light(wavelength=about 660 nm). An example of such a situation is describedin the Japanese patent JP2000076679, where disc-tilt is measured fromthe position of a spot with a light source having a wavelength differentfrom the wavelength used for reading out the information on the disc. Itturns out, however, that, when such a method is applied, the size of thespot on the optical detector is now too large compared to the size ofthe standard four-quadrant detector conventionally used in optical discdrive apparatuses: no significant difference in the signals from thequadrants of the detector can be measured, and hence no tilt errorsignal can be derived.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide an optical discdrive avoiding this drawback.

To this end, the invention relates to an optical disc drive as definedin the introductory paragraph of the description and in which theoptical scanning means themselves comprise at least:

-   -   a first light source, for producing said first light beam;    -   focusing means, applied to said light beam and provided between        said first light source and a focusing point on an information        layer on said first disc having a first cover layer;    -   an optical detector provided for receiving a first backward beam        reflected from said information layer of said first disc;    -   a second light source for producing a second light beam also        transmitted to said focusing means and for measuring tilt from        the position, on said optical detector, of a second spot        corresponding to a second backward beam obtained after        reflection of said second light beam on said information layer        of said first disc;        said optical disc drive further comprising, between said        focusing point and said optical detector, a diffractive        structure provided with diffracting elements for substantially        refocusing the returning second beam onto the detector.

This structure allows to generate on the detector a spot small enough inorder to be able to measure significant differences between the signalsof the detector and therefore to derive a noticeable tilt error signal.

Preferably, the diffractive structure is attached to one surface of aservo-lens positioned just before the optical detector. However, it mayalso be attached to one surface of an objective lens used as focusingmeans, or to a separate plate.

In an advantageous implementation, the diffractive structure consists ofa series of ring-shaped prisms, but it can also be approximated by astep-wise structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, in a moredetailed manner and with reference to the accompanying drawings, inwhich:

FIG. 1 illustrates an example of optical disc drive suitable for storinginformation on or reading information from an optical disc;

FIG. 2 shows an example of optical scanning device;

FIGS. 3 and 4 show two views (from the top and in cross section,respectively) of a first implementation of the diffracting structureaccording to the invention;

FIG. 5 shows a phase-step structure that is an approximation of theprism structure illustrated in the cross section of FIG. 4;

FIGS. 6 and 7 illustrate the size and localization of the spot,respectively without and with the diffractive structure according to theinvention;

FIG. 8 is a graph showing a simulated normalized radial-tilterror-signal obtained in the case of the implementation of FIGS. 3 and4;

FIG. 9 is a graph showing a similar normalized radial-tilt error-signalobtained in the case of the implementation of FIG. 5;

FIG. 10 shows how the light passes through the objective lens and isfocussed on the information layer, and FIG. 11 is a magnified view ofthe region around said information layer.

DETAILED DESCRIPTION OF THE INVENTION

An example of an optical disc drive, suitable for storing information onor reading information from an optical disc 1, is schematicallyillustrated in FIG. 1. The disc drive comprises an apparatus frame 2and, for rotating the disc 1, a motor 3 fixed to the frame 2 anddefining a rotation axis 4. For receiving and holding the disc 1, thedisc drive may comprise a turntable or clamping hub 5, which is mountedon the axle 6 of the motor 3. The disc drive also comprises a sledge 7,which is displaceably guided in the radial direction of the disc 1, i.e.in a direction substantially perpendicular to the rotation axis 4, byguiding means not shown for the sake of clarity. A radial sledgeactuator 8 is provided for regulating the radial position of the sledge7 with respect to the apparatus frame 2.

The disc drive further comprises a platform 9, which is displaceable inthe radial direction of the disc 1 with respect to the sledge 7, andwhich is displaceably mounted with respect to the sledge 7 by mountingmeans not shown for the sake of clarity. A radial platform actuator 10is provided for radially displacing the platform 9 with respect to thesledge 7. Radial sledge actuators and radial platform actuators areknown per se, and as the design and operation of such radial platformactuator are not the subject of the present invention, it is notnecessary here to discuss them in great detail.

The disc drive further comprises, for scanning the tracks (not shown) ofthe disc 1 by means of an optical beam, an optical device 20 which isdescribed later. It also comprises a pivot platform actuator 11,arranged for pivoting the platform 9 with respect to the sledge 7 (suchpivot platform actuators are known per se and therefore not discussed ingreat detail), and a control unit 12 having the following outputs: afirst output 12 a connected to a control input of the motor 3, a secondoutput 12 b coupled to a control input of the radial sledge actuator 8,a third output 12 c coupled to a control input of the radial platformactuator 10, a fourth output 12 d coupled to a control input of an axialplatform actuator 13, and a fifth output 12 e coupled to a control inputof the pivot platform actuator 11. The control unit 12 is designed togenerate at said outputs 12 a to 12 e control signals S_(CM), S_(CS),S_(Cpr), S_(Cpa) and S_(CPp) respectively for controlling the motor 3,the sledge actuator 8, the radial platform actuator 10, the axialplatform actuator 13 and the pivot platform actuator 11. The controlunit 12 has also a read signal input 12 f for receiving a read signalS_(R) from an optical detector described in the following paragraphs.

The optical scanning device 20 will now be described. According to apreferred embodiment of the invention illustrated in FIG. 2, the opticalscanning device comprises light beam generation means 21 (first lightbeam), typically a laser, which may be mounted with respect to the frame2 or the sledge 7. This laser 21 produces a diverging radiation beam 22of a given first wavelength (in this example 660 nm), and the linearlypolarized light flux thus emitted is reflected by a semi transparentmirror 23 and transmitted through a polarized beam-splitter 24 towards amirror 25 and a lens system. This lens system includes a collimator lens26, that changes the diverging radiation beam 22 to a collimatedradiation beam 28, and an objective lens 27, that transforms saidcollimated beam into a converging beam 29 which comes to a focus 30 onthe information layer of the disc.

After its reflection on said information layer, the converging beam 29forms a reflected beam which returns on the same optical path. Thisbackward radiation is further transmitted through the polarizedbeam-splitter 24 and passes through a servo-lens 40 which transformssaid returning beam into a converging beam 41 falling on an opticaldetector such as a servo-detector 42. The semi-transparent mirror 23 isconstructed with optical elements that allow to separate the emittedbeam and the returning one and to supply the latter to the opticalelements (40,42) that follow said mirror.

In order to obtain a measure of the amount of radial and tangential tiltof the disc 1, a second light source 51, typically a laser with a secondwavelength different from said first wavelength (in this example 780nm), is temporarily switched on for producing a second light beam. Bydefinition, the two wavelengths are considered as different when theabsolute difference between them is more than 10 nm. The polarized lightemitted by said light source 51 is reflected by the polarizedbeam-splitter 24 and the mirror 25, and then passed through thecollimator lens 26 and focused by the objective lens 27. A secondbackward beam obtained after reflection of said second beam on theinformation layer is then received by the servo-detector 42. Since thewavelength of the second light beam is different from that of said firstlight beam, the spot on the information layer of said first disc is nowaberrated. As a result the size of the spot on the detector of thissecond beam becomes larger than the size of the detector.

According to the invention, it is proposed, in order to overcome theproblem of the size of the spot on the servo-detector, to attach to onesurface of one of the lenses located on the path of the backward beam oron a separate plate, between the focusing point on the information layerand the servo-detector, a diffractive structure implemented as describedhereinunder. By doing so, the size of the spot on the servo-detector isreduced when accessing said first disc with the light emitted by thesecond light source.

An example of this diffractive structure is shown in FIGS. 3 and 4. Forthe second light beam produced by radiation source 51, the diffractivestructure should diffract the light beam substantially in one particulardiffraction order. In this example, the case where the light isdiffracted into the first diffraction order is considered. This can beachieved by a diffractive structure 100 which is of a blazed gratingtype and consists of a series of ring-shaped prisms 101, as seen fromthe top (in FIG. 3) and in cross-section along the radius (in FIG. 4).In the cross-section of FIG. 4, the numbers indicate the radial positionof each of the diffracting elements, in millimetres. The height h₂ ofthe prisms is given by:h ₂=λ₂/(n ₂−1)   (1)

where λ₂ is the wavelength of the light emitted by the second lightsource 51 and n₂ is the refractive index of the prism material at thewavelength λ₂. As shown in FIGS. 3 and 4, the central region of thediffractive structure (until a radius of 0.154 mm) does not contain anydiffracting element. The size of this region is chosen so that whenaccessing a second disc having a second cover layer with light from thesecond light source 51, the reflected light passes the structure 100unaltered, while, when accessing said first disc having said first coverlayer with light from said second light source, a substantial amount ofthe rays of the divergent returning beam that fall outside the detector42 are re-focused onto said detector. The following table 1 givestypical values for the radius (in millimetres) of the successive ringsconstituting the diffracting structure, together with the pitch, i.e.the distance between two successive rings, in micrometres: TABLE 1 Ringnumber Radius (mm) Pitch (μm) 1 0.154 0.0 2 0.217 63.2 3 0.265 48.0 40.306 40.1 5 0.340 35.0 6 0.372 31.3 7 0.400 28.6 8 0.427 26.4 9 0.45123.0 10 0.474 21.7 11 0.496 20.6 12 0.517 19.6

An important feature of the diffractive structure is that it should notinfluence the returning light of said first light beam when this lightis used to access said first disc. A blazed grating however alsodiffracts the first light beam significantly into the first diffractionorder. In order to prevent this, instead of a blazed grating a binarygrating must be used. In a binary grating, each blaze is approximated bya step-wise structure (see FIG. 5). The international patent applicationWO02/21522 describes how to design such binary gratings diffracting asubstantial part of the second light beam to the first diffraction orderwhile not influencing the first light beam. The methods explained insaid patent application are therefore included by reference. This resultmay be obtained with the proposed step-wise structure of FIG. 5 wherethe blazed grating is approximates with four steps (105, 106, 107 and108). It can be shown that with this structure, the light emitted by thefirst light source is unaltered by the grating, because the step heightshave been chosen as multiples of h₁, each h₁ step leading to a phaseshift of 2 π when using light from the first light source (the phaseshift for light emitted by the second light source is generally not 2π). The step height h₁ is given by:h ₁=λ₁/(n ₁−1)   (2)where λ₁ is the wavelength of the light emitted by the first lightsource and n₁ is the refractive index of the grating material at thewavelength λ₁. Here, for exemplary purposes, the refractive index of thegrating material for both λ₁ and λ₂ is assumed to be 1.65. For thesituation shown in FIG. 5, the height of step 105 is 1×h₁, the height ofstep 106 is 2×h1, the height of step 107 is 4×h1 and the height of step108 is 5×h1. The diffraction efficiency of the shown step structure isapproximately 75 per cent.

The diffracting structure used, according to the invention, to reducethe size of the spot from the second light source on the servo-detectoris either attached to the servo-lens or to the objective lens or on aseparate plate. In the first case, the diffractive structure (such asillustrated in FIG. 5) is attached to the front surface of theservo-lens, and FIGS. 6 and 7 show respectively the spot 110 without thediffracture structure and the spot 111 with the diffractive structureattached to the servo-lens. As can be seen from FIG. 7 compared to FIG.6, the spot size is reduced as compared to the situation without thediffracting structure (in these figures, the black squares 112 indicatethe size of the standard four-quadrant detector used in optical pick-updevices). At least more than 10% of the light of said second beamfalling outside the detector when the grating according to the inventionis not present is not refocused onto the detector when the grating ispresent. More preferably, more than 25% is refocused onto the detector.The normalized tilt error-signal is derived from the position of thespot from the second light source on said four-quadrant detector. If A,B, C and D refer to the individual quadrants, the normalized radial-tilterror signal, called RTES, is given by: $\begin{matrix}{{RTES} = \frac{A + B - C - D}{A + B + C + D}} & (3)\end{matrix}$The simulated normalized radial-tilt error-signal obtained is shown inFIG. 8.

In the second case, the diffractive structure may be attached to theobjective lens. The radius and the corresponding pitch for each ring arethen given in the following table 2, together with the numericalaperture (NA) belonging to the radius: TABLE 2 Ring number Radius (mm)Pitch(μm) NA 1 1.360 0.0 0.49 2 1.624 264.0 0.59 3 1.799 175.0 0.65In order not to influence the returning light from the second lightsource when accessing a second disc having said second cover layer, thediffractive structure is confined to the region defined by 0.5<NA<0.65,where it acts as a weak positive lens for the light emitted by thesecond light source. Due to the lens effect, the rays are re-focused onthe servo-detector (it may be noted that the diffractive structure hasan overall aspheric shape in order to fit onto the aspheric surface ofthe objective lens). As described above, the grating structure may beapproximated by a phase-step profile, so that the light from the firstlight source is not affected when passing through the region0.5<NA<0.65. The normalised radial-tilt error-signal obtained with thismodified objective lens is shown in FIG. 9. The actual focus position ofthe edge rays (i.e. the part of the light from the second light sourcethat falls in the region 0.5<NA<0.65 on its way to the disc) isimportant. This is shown in FIGS. 10 and 11, where FIG. 10 shows thelight from the second light source that passes through the objectivelens (OL) 27 and is focused onto the information layer 120 of the disc 1(note that FIG. 10 shows an unfolded light path, i.e. the system isshown in transmission, while FIG. 11 is a magnified view of the regionaround the information layer 120). As can be seen, the actual focus 121is positioned “behind” the information layer 120. It is important thatthe diffractive structure brings the edge rays 122 to a focus 123 infront of this point and only slightly behind the information layer 120.Focusing the edge rays onto the information layer suppresses the beamlanding as a consequence of disc tilt and hence does not lead to auseful error-signal.

It must be understood that the invention thus defined is not limited tothe above-mentioned implementations. Whilst in the above describedembodiment the first wavelength is smaller than the second wavelength,it is possible that the first wavelength may also be larger than thesecond wavelength. Also, whilst in the above-described embodiment,explicit reference is made to a detector with four quadrants, it appearsthat the principle can be applied to a detector having at least two oreven six and more segments. Although the embodiment described aboveindicates a situation where the beam from the second light source usedfor the tilt measurement is confined to a numerical aperture which issmaller than the numerical aperture of the beam from the first lightsource, it is appreciated that a similar principle to the one givenabove can be applied to the opposite situation.

1. An optical disc drive comprising rotating means, defining a rotatingaxis for an optical disc, and optical scanning means, for scanning saidoptical disc with a light beam, said optical scanning means themselvescomprising at least: a first light source, for producing said firstlight beam; focusing means, applied to said light beam and providedbetween said first light source and a focusing point on an informationlayer on said first disc having a first cover layer; an optical detectorprovided for receiving a first backward beam reflected from saidinformation layer of said first disc; a second light source forproducing a second light beam also transmitted to said focusing meansand for measuring tilt from the position, on said optical detector, of asecond spot corresponding to a second backward beam obtained afterreflection of said second light beam on said information layer of saidfirst disc; said optical disc drive further comprising, between saidfocusing point and said optical detector, a diffractive structureprovided with diffracting elements for substantially refocusing thereturning second beam onto the detector.
 2. An optical disc driveaccording to claim 1, in which said diffractive structure is attached toone surface of a servo-lens positioned just before said opticaldetector.
 3. An optical disc drive according to claim 1, in which saiddiffractive structure is attached to one surface of an objective lensused as focusing means.
 4. An optical disc drive according to claim 1,in which said diffractive structure is attached to a separate plate. 5.An optical disc drive according to claim 2, in which said diffractivestructure consists of a series of ring-shaped prisms.
 6. An optical discdrive according to claim 2, in which the diffractive structure isapproximated by a step-wise structure.